Fixes #2658.

CBaseListView, CGridView, CListView: added note about $itemsCssClass and
$pagerCssClass properties, they must not contain empty string, null or
false values.
'CJoinQuery::createCommand()' fixed to exclude duplicating joins.
Issue #2970 added to CHANGELOG
Merge pull request #2977 from klimov-paul/2970-ar-duplicate-joins

Fix for #2970 Yii doesn't track what tables are connected
Typo fix in PHPDoc
Merge pull request #2989 from munawer-t/master

Typo fix in PHPDoc